Avalanche Forecast
Regions: Olympics.
Very dangerous avalanche conditions should continue Saturday in the mountains. Sustained very heavy snowfall since noon on Thursday finally relents early Saturday after creating storm slabs that need time to heal and may step down to weaker interfaces. Backcountry travel above treeline is not recommended while near treeline and into the upper treeline band, you should evaluate all slopes 30 degrees and steeper.
Discussion
The Hurricane Ridge Road crew reported a lot of small natural Loose Wet natural avalanches on Friday morning.
As of 4 PM on Friday, Hurricane Ridge had blown all model projections of precipitation amounts out of the water (literally), with 3.07†of snow water equivalent since the storm began late-morning on Thursday. This has come as all snow at Hurricane Ridge proper with snow levels rising to a peak on Thursday evening around 4000-4500 ft, before dropping to around 3300 ft by Friday afternoon.Â
The terrain is really starting to fill in quickly at Hurricane Ridge with average total snow depths likely 30-45†near treeline by Saturday morning. The rapid change loads a shallow early season snowpack and stresses several notable weak interfaces identified on Thursday: (1) On northerly aspects, a layer of 3-4 mm well-preserved surface hoar and (2) on southerly aspects, a strong 5 cm crust. Both these planes of weakness showed reactivity in tests and may contribute to large and more widely propagating avalanches than you expect.Â
Huge changes to our snowpack bring great uncertainty in the avalanche danger. That uncertainty should limit the types of terrain you are willing to travel on Saturday, taking convex rolls and steeper terrain off the menu. Please submit an observation to NWAC and tell us what you saw out there.
Forecast Schedule
For the 2019-20 winter season, avalanche danger ratings will be issued for the Olympics every Friday through Sunday and during mid-week holidays. During the week, No Rating will be issued but forecasts will include expected conditions and relevant travel advice. If you are out in the Olympics, share your backcountry observations with us and the greater community.
Snowpack Discussion
December 19th, 2019Â (The regional synopsis is updated every Thursday @ 6 pm)
Let’s take a moment to recap what happened over the past week:
A potent winter storm arrived on the 12th, adding to the very shallow snowpack throughout the region. Anywhere from 6-36†of snow fell between the 11th and 15th. Places like Mt Baker, Paradise, Mt Hood, and White Pass were the winners regarding snowfall, with quite a bit less for areas further east of the crest like Mission Ridge, Blewett Pass, and Washington Pass.Â
The most notable avalanche activity occurred in the Mt. Baker backcountry where numerous human triggered storm slab avalanches occurred (with several big enough to injure, bury, or kill a person). The majority of these occurred on upper elevation, north and east facing terrain. Besides the danger posed by the size of the avalanche, many slides quickly revealed rocks and other obstacles barely hidden below the snow surface.Â
The storm layer took time bond with the underlying snow surface, and we saw the likelihood of triggering slab avalanches slowly decrease over a handful of days. The avalanche danger was at Considerable for many places on Saturday the 14th, then tapered to Moderate on Sunday, and eventually reached Low in many places by Tuesday and Wednesday.Â
A large, human triggered avalanche on the old snow interface. East aspect of Table Mountain (West-North Zone) at 5,500ft. December 15,2019. Photo by Brooks Broom.Â
Backcountry travelers have noted generally thin and shallow snowpacks, with a range of structures across the region. Many folks were skeptical of the layering they found. People were able to dig down and observe weak layers in many areas. These were buried in late November and early December. In some places they presented as weak snow over a crust, in others, a layer of feather-like surface hoar. Cold temperatures have likely preserved many of these layers, and will be worth considering as the snow piles up.
Below is a list of commonly visited locations and their snowpack depths in inches (as of December 19th at 4am). Check the weather station hourly data feed and watch as the subtropical javelin of moisture (also known as a strong atmospheric river) changes conditions dramatically over the next few days.Â
Total Snow Depth in Inches as of December 19th at 04:00 AM.
-Matt Primomo
Avalanche Problems
Storm Slabs
Recent substantial snowfall, heavy precipitation rates, winds, and fluctuating freezing levels create the perfect recipe for avalanches Saturday. Avalanches could grow large, step down into older snow layers, and travel to lower elevations. The storm should wane by early Saturday, but the avalanche danger will remain high, particularly at upper elevations where it was colder. We are particularly concerned about northerly aspects where a layer of buried surface hoar may be found in wind-sheltered mid-slope locations. Until you check multiple locations for this layer, avoid slopes steeper than 30 degrees or shift your aspect.
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Very Likely
Expected Size: 1 - 1